Second generation mobile communication refers to transmission and reception of voice into digital and is represented by Code Division Multiple Access (CDMA), Global System for Mobile communication (GSM) and the like. General Packet Radio Service (GPRS) was evolved from the GSM. The GPRS is a technology for providing a packet switched data service based on the GSP system.
Third Generation mobile communication refers to transmission and reception of image and data as well as voice (audio). Third Generation Partnership Project (3GPP) has developed a mobile communication system (i.e., International Mobile Telecommunications (IMT-2000)), and adapted Wideband-CDMA (WCDMA) as Radio Access Technology (RAT). The IMT-200 and, the RAT, for example, the WCDMA are called as Universal Mobile Telecommunication System (UMTS) in Europe. Here, UTRAN is an abbreviation of UMTS Terrestrial Radio Access Network.
A common cellular network constructs a radio communication link with high inter-reliability by a central cell design that communications are enabled via a direct link between a base station and a mobile terminal (mobile station) within a cell that the base station covers. However, service frequency ranges are getting extended in the recent communication networks and radiuses of cells are gradually decreased for supporting (covering) high-speed communication and more traffic. Thus, many problems may be caused in view of applying the conventional centralized cellular radio network as it is even later. That is, since a position of the base station is fixed, flexibility of a radio link configuration is low. As a result, it has been difficult to efficiently provide communication services in a radio (wireless) environment where there is a rapid change in traffic distribution and call demands.
Therefore, the next generation communication system should be distributively controlled and constructed, and also actively meet environmental changes, such as addition of a new base station.
To address this shortcoming, a multi-hop relay system has been introduced. Advantageously, the multi-hop relay system can broaden cell service coverage by covering a partial shadow area generated within a cell region, increase system capacity, and reduce an initial installation charge since a relay (hereinafter, referred to as ‘relay station (RS)’) is established in an initial stage when a service request is not frequently made. The relay system may relay a signal of a mobile station (MS) to a base station (BS) via a relay station (RS) when the MS is located far away from the BS or a signal transmission is not smoothly performed due to an obstacle such as buildings or the like, thereby broadening the cell coverage and solving the shadow area.
FIG. 1 illustrates a multi-hop relay system.
As illustrated in FIG. 1, an MS 11 within the coverage area of a BS 30 is connected to the BS 30 through a direct link, and an MS 12, which is located outside the coverage area of the BS 30 and thus has a poor channel quality with respect to the BS 30, is connected to the BS 30 through an RS 20.
Here, the RS 20 is located on a roof of a building, relatively higher than the MS 10, and fixedly installed thereat. Hence, the channel environment between the RS 20 and the BS 30 has less change than the channel between the MS 11 and the RS 20, and achieves a high channel gain on the average.
Since the RS is established between the BS and the MS for signal relay, a BS-MS link, a BS-RS link and an RS-MS link are established. Such link is referred to as hop. That is, when the MS is located in a cell boundary region of the BS or in a shadow area suffering a serious shielding phenomenon due to buildings, the MS may perform communications with the BS through the RS. Thus, by using the relay scheme, the BS can provide a high-speed data channel in the cell boundary region in a poor channel quality and also can extend the cell service coverage.
A plurality of RSs may be present according to a channel between the MS and the BS or the RS and other conditions, which is referred to as ‘multi-hop relay system.’
In the multi-hop relay system, when a plurality of RSs are sequentially connected, a relay station (RS) which controls and supports subordinately connected MS and RS is referred to as a superordinate RS, and a relay station (RS) which is connected to the superordinate RS and supports the MS is referred to as a subordinate RS.
As such, the relay-employed environments are discussed even in the WiMAX standard as the next generation mobile communication system.
The WiMAX standard has adapted introduction of the RS, and discussed a centralized scheduling-based multi access scheme that the BS directly performs scheduling and a distributed scheduling-based multi access scheme that the BS and the RS distributively perform scheduling, respectively.
FIG. 2 illustrates an exemplary frame structure used in the multi-hop relay system.
Referring to FIG. 2, a superframe is divided into four radio frames each having the same size. The superframe may include a superframe header. The superframe header may include essential control information that an MS should acquire upon an initial network entry or handover, and function similar to a Broadcast Channel (BCH) in the LTE technology. The superframe header may be assigned to a first radio frame of a plurality of radio frames constituting a superframe. The number of subframes constituting one frame may be variable to 5, 6, 7 or 8 depending on a bandwidth of a system or a length of a cyclic prefix (CP), and the number of symbols of OFDMA constituting one subframe may also be variable to 5, 6, 7 or 9. FIG. 2 exemplarily illustrates that the length of CP is ⅛ Tb (Tb: Useful OFDMA symbol time) when a bandwidth is 5, 10 or 20 MHz.
The frame structure exemplarily illustrated in FIG. 2 may be applied to a Time Division Duplexing (TDD) or Frequency Division Duplexing (FDD) scheme. In the TDD, an entire frequency band is used for uplink (UL) or downlink (DL) transmission but is divided into UL transmission and DL transmission at a time domain. In the FDD scheme, UL transmission and DL transmission occupy different frequency bands and can be simultaneously performed.
FIG. 3 illustrates a relay frame structure in accordance with one exemplary embodiment.
The frame structure illustrated in FIG. 3, is illustrated based on one example among structures proposed in IEEE 802.16.
According to the illustrated frame structure, an RS, a BS and an MS may perform merely unidirectional transmission or reception at the same time domain, which is referred to as a unidirectional mode. Also, regarding the illustrated frame structure, RSs are divided into an odd-hop RS and an even-hop RS according to the number of hops from the BS.
A DL subframe of an odd-hop RS may include 16 m transmit zone and 16 m DL receive zone in order, and a UL resource of the odd-hop RS may include 16 m UL receive zone and 16 m UL transmit zone in order.
A DL subframe of an even-hop RS may include 16 m DL receive zone and 16 m transmit zone, and a UL subframe of the even-hop RS may include 16 m UL transmit zone and 16 m UL receive zone in order.
The 16 m DL transmit zone is a DL period of an RS, and the RS may perform transmission to a subordinate RS and an MS.
The 16 m DL receive zone is a DL period of an RS, and the RS may perform reception from a superordinate RS.
The 16 m UL transmit zone is a UL period of an RS, and the RS may perform transmission to a superordinate RS and an MS.
The 16 m UL receive zone is a UL period of the RS, and the RS may perform reception from a subordinate RS and an MS.
Meanwhile, a DL subframe of a BS may include a zone between RS and MS (indicates by BS□MS) and 16 m DL relay zone. The DL relay zone is a DL period of the BS, and the BS may perform transmission to an RS and an MS. A UL subframe of the BS may include a zone between RS and MS (indicated by BS□MS) and a UL relay zone. The UL relay zone is a UL period of the BS, and the BS may perform reception from an RS and an MS.
FIG. 4 illustrates another exemplary relay frame structure.
The frame structure illustrated in FIG. 4 is illustrated based on another example among the structures proposed in IEEE 802.16.
According to the illustrated frame structure, an RS may perform transmission or reception bi-directionally, namely, to or from a BS and a subordinate RS at the same time domain. This is referred to as a bi-directional mode. The relay frame structure may include a bi-directional transmit zone and a bi-directional receive zone. In the bi-directional transmit zone, the RS may perform transmission to a superordinate RS/BS and a subordinate RS. In the bi-directional receive zone, the RS may receive data from a subordinate RS and a superordinate RS/BS.
In 16 m DL access zone as illustrated in FIG. 4, a BS or RS may perform transmission to an MS. In 16 m UL access zone, a BS or RS may receive data from an MS.
FIG. 5 illustrates another exemplary relay frame structure.
The frame structure illustrated in FIG. 5 is illustrated according to a connection among a BS, a non-transparent relay and a transparent relay.
The transparent RS may not generate and transmit its own control signal, for example, preamble, FCH, MAP, DCD/UCD, SFH, etc. On the other hand, the non-transparent RS may directly generate its own control signal for transmission.
The transparent RS may be connected to a BS, and the non-transparent RS may be subordinately connected to the transparent RS.